Wireless communication is rapidly coming into common use, as a technology whereby the burden of wiring work of conventional cable communication can be resolved, and further, whereby mobile communication can be realized. Examples of typical standards relating to wireless LANs (Local Area Network) include IEEE (The Institute of Electrical and Electronics Engineers) 802.11 and IEEE 802.15.
Many wireless LAN systems use access control procedures based on carrier sense such as CSMA/CA (Carrier Sense Multiple Access with Collision Avoidance: Carrier Sense Multiple Access) so that individual communication stations can avoid carrier collision when performing random channel access.
FIG. 22 illustrates the way in which three communication stations STA-0, STA-1, and STA-1 each operating in its own communicable range. Also, FIG. 23 illustrates an example of a communication sequence based on CSMA/MA under a communication environment where three communication stations STA-0, STA-1, and STA-1 are operating. A communication station where a transmission request has occurred monitors the media state for a predetermined frame interval DIFS (Distributed Inter Frame Space), and if there is no transmission signal present during this time performs random backoff, and if there is no transmission signal present during even this time, obtains a transmission right and can transmit a frame.
Also, communication stations are permitted to transmit a frame (packet) after a shorter frame interval SIFS (Short Inter Frame Space) in the event of transmitting a frame with exceptionally high urgency such as an ACK. Accordingly, a frame with high urgency can be transmitted before a frame regarding which transmission performed following normal CSMA procedures.
Now, with wireless communication, there is known the occurrence of a problem called hidden node problem, in which there is a region where communication stations cannot directly communicate one with another. Hidden nodes cannot perform negotiation with each other, so there is the possibility that transmission operations will collide. One methodology for solving the hidden node problem is “virtual carrier sense”. Specifically, in the event that Duration (elapsed time) information for reserving media is described within a received frame which is not addressed to itself, an assumption, i.e., virtual carrier sensing, is made that the media will be in used for the period corresponding to the Duration information, and a transmission stop period (NAV: Network Allocation Vector) is set.
Further, a typical example of a signal transmission/reception sequence using virtual carrier sense is an RTS/CTS handshake. The communication state which is the data originator transmits a transmission request frame (RTS: Request To Send), and in response to having received a confirmation notification frame (CTS: Clear To Send) from the communication station at the data destination, data transmission is started. Upon a hidden node receiving at least one frame of an RTS or CTS which is not addressed to itself, the transmission stop period is set based on the Duration information described in the received frame, so as to avoid collision. A hidden node as seen from the transmission station receives a CTS and sets a transmission stop period to avoid collusion with a data frame, and a hidden node as seen from the receiving station receives an RTS and stop transmission period to avoid collision with an ACK.
Using RTS/CTS handshake along with CSMA/CA control procedures can in some instances reduce overhead of collision in an overloaded state.
Now, array antenna technology is an example of a method for securing a good communication channel with a particular communication party. The beam pattern can be controlled by changing the transmission weight or reception weight for each antenna, and by orienting at least one of the transmission beam and reception beam in the direction in which the communication party is situated improves quality of the communication channel.
For example, a proposal has been made to improve antenna gain by performing transmission/reception with a base station forming a unique beam pattern for each mobile station, so as to provide a good communication path (e.g., see Patent Document 1).
Also, a proposal has been made regarding a communication method wherein the base station controls the directionality of the antenna when transmitting data signals but does not control directionality of the antenna when transmitting synchronization signals, and performs suitable processing in the event of multiplexed data signals existing in the same time period as with synchronization signals such as reducing the length of the synchronization signals, thereby effecting interference mitigation regarding a terminal device before synchronization has been established (e.g., see Patent Document 2).
FIG. 24 illustrates an example of a communication environment using array antenna technology. With the communication environment shown in FIG. 22, the communication stations STA-0, STA-1, and STA-2 do not generate a beam pattern addressed to a particular communication station, and each perform nondirectional (omni directional) transmission/reception with beam patterns Beam-1, Beam-1, and Beam-2. On the other hand, with the example shown in FIG. 24, the STA-0 which serves as the base station generates beam patterns with higher gain as to particular communication stations STA-1 and STA-2 to perform communication, such as Beam-01 and Beam-02 as to communication parties STA-1 and STA-2, respectively (in other words, focused onto STA-1 and STA-2). STA-1 and STA-2 serving as terminal stations each have nondirectional beam patterns Beam-1 and Beam-2, but the STA-0 transmits with the beam patterns Beam-01 and Beam-02 directed to each station. Accordingly, the reception SINR (Signal-to-Interference plus Noise power Ratio) at the STA-1 and STA-2 improves.
Also, a communication method employing the directionality of antennas is also applied to IEEE 802.15.3c which is a standard for wireless PAN (mmWPAN: millimeter-wave Wireless Personal Area Network) using millimeter wavebands. Millimeter wavebands have short wavelength and strong linearity even in comparison with microwaves widely used with wireless LAN technology, and can transmit very great amounts of information, but attenuation due to reflection is marked, and propagation loss is great, so wireless signals do not reach far. The range problem of millimeter wavebands can be supplemented by transmission/reception beam pattern control.
The beam pattern is generally calculated based on transmission channel information with a communication party. Accordingly, at the time of controlling beam patterns, reception signals from the communication party are necessary. With the communication environment operated with the STA-0 serving as the base station, the subordinate terminal stations STA-1 and STA-2 each periodically transmitting signals (signal) for updating (refreshing) the beam pattern to the STA-0 is a prerequisite. Also, in the event of performing transmission/reception of signals with a particular beam pattern, signal detection using a preamble is not performed. Preamble detection is performed only in the case of random channel access such as shown in FIG. 23, and transmission/reception for random channel access is performed with a different beam pattern as transmission/reception of data frames. For example, with the communication system shown in FIG. 24, the base station STA-0 uses beam patterns Beam-01 and Beam-02 directed toward the communication parties when performing transmission/reception of data frames with STA-1 and STA-2, but uses nondirectional beam pattern Beam-0 such as shown in FIG. 22 when performing random channel access.
Communication stations need to perform preamble detection at the time of performing access control based on physical carrier sense, in order to receive signals from unidentified nearby stations. On the other hand, in the event of performing communication where communication with a particular communication station is difficult unless a beam pattern is formed, performing access control based on physical carrier sense becomes difficult since signals from unidentified stations cannot be received if a beam pattern is formed.
With a system such as according to the aforementioned Patent Documents 1 and 2 where communication is performed with beam patterns for particular communication stations under a base station and beam patterns for unidentified communication stations being separately formed, there is no particular problem for performing channel sharing without access control based on physical carrier sense. On the other hand, in a case such as the airwave bandwidth being an unlicensed band, channel sharing by access control based on physical carrier sense is desirable. For example, with a private network such as a wireless LAN using millimeter wavebands, it is thought that there is the need to use access control based on physical carrier sense and beam pattern control, in tandem.